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Effect of multiple reflow processes on the reliability of ball grid array (BGA) solder joints
Journal of Alloys and Compounds 414 (2006) 123–130
Effect of multiple reflow processes on the reliability of ball grid array (BGA) solder joints W.H. Zhong a , Y.C. Chan a,∗ , M.O. Alam a , B.Y. Wu a , J.F. Guan b a
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, PR China b Department of Mineral Processing, Wuhan University of Science & Technology, Wuhan, PR China Received 26 June 2005; received in revised form 20 July 2005; accepted 22 July 2005 Available online 15 February 2006
Abstract This paper studies the mechanical behaviors and microstructures of the ball grid array (BGA) solder joints against OSP-coated Cu pads on FR4 substrates after multiple reflow process. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with the popular Pb-free solder Sn–3Ag–0.5Cu (SAC), and the traditional Sn–37Pb (SP) solder. It is found that SACI solder joints provide higher shearing strength over SAC and SP solder joints during multiple reflow. The shearing force of the three types of solder joints does not have significant change with the increase of the number of reflow cycles. The fracture of all SAC and SP solder joints mainly occurs in the bulk solder, whereas the SACI solder joints show a diverse manner, with various combinations of bulk solder facture, solder/IMC interface dissociation, and pad/resin interface failure. The differences in mechanical behaviors are interpreted in terms of diffusion and interaction, as well as chemical and thermal induced degradation. The microstructure evolutions of the solder joints during multiple reflow soldering are also examined. A continuous change of chemical compositions in the IMC layer/particles has been noticed. Large-shaped Ag–In–Sn phase near the IMC layer has been found within the SACI solder joints on the Cu pads. © 2005 Elsevier B.V. All rights reserved. Keywords: Multiple reflow; Pb-free; Ball grid array solder joints; Shearing force; Microstructure
1. Introduction Due to worldwide environmental concerns, public sentiments and market strategies, as well as governmental regulations on the use of toxic Pb in the electronic products, the conventional Sn–Pb solders are under strict scrutiny. Therefore, many Sn-rich alloy systems have been developed as alternative candidate Pb-free solders, which have attracted much researches and trial applications in recent years [1]. With combination of process attributes (modest melting point and reasonable solderability), comparable electrical performances and good mechanical properties, the Sn–Ag–Cu system is one of the potential choices recommended by some organizations, such as consortium of National Physics Laboratory (NPL), International Tin Research Institute (ITRI) ∗
Corresponding author. Tel.: +852 2788 7130; fax: +852 2788 7579. E-mail address: [email protected] (Y.C. Chan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.047
and Department of Trade & Industry (DTI) of UK, National Electronics Manufacturing Initiative (NEMI) and National Center for Manufacturing Sciences (NCMS) of USA, as well as Japan Electronics and Information Technologies Industries Association (JEITA), etc. [2–3]. Although great effort has been made for more than 10 years to get the Pb out of electronic industry, until now, there are still some tough barriers needed to override for a widespread and uniform industrial acceptance of favorable solder alloys and matched materials plus processes [4–5]. Besides the lack of legislation, the high cost and insufficient backup data of Pb-free operation (compared to the well-established Sn–Pb soldering system) are cautious consideration nowadays. Moreover, due to the variation of the reflow temperature and materials, another challenge to massive Pb-free application is the uncertainty of many reliability issues with thermal damage, mechanical degradation, tin whisker growth and extensive solder/pad interfacial reaction.
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Fig. 2. Schematic drawing of a solder joint and shearing test setup. Fig. 1. Variation of average shearing force of solder joints as a function of the number of reflow cycles.
So, many individuals and organizations have been making efforts to promote the Pb-free implementation [6–9]. Recently, Hwang et al. has invented a novel solder alloy with the addition of In element and claimed that
Sn–Ag–Cu–In system possesses low melting temperature range, which can achieve Pb drop-off without changing the conventional reflow temperature and related materials and equipments [10]. This may cut off the overall manufacturing cost because of the less modification of the present facili-
Fig. 3. Typical shearing results of the SP solder joints: (a) low magnification image capturing two solder joints, (b) a fracture surface on the pad side and (c) a cross-sectional picture of the pad after shearing test.
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ties and technologies. Furthermore, it can also benefit the perspective of Pb-free application in the high-demanding packages such as the flip chip (FC) and ball grid array (BGA) technology, which can reduce the reliability problems due to low-temperature process attributes. The BGA technology becomes more and more popular by the virtue of enhancing miniaturization, performance and functionality. As the high-density packaging develops towards finer pitch and smaller size, the importance of mechanical integrity of the solder joints is essentially emphasized to the reliability concerns. So far quite a lot of work on mechanical strengths of the BGA solder joints has been done based on different solders, pad and substrate design, shearing/pulling parameters, reflow ambience and aging conditions, while only a few works were focused on the effect of multiple reflow process [11–21]. However, the BGA solder joints may experience several times of reflow during component manufacturing, soldering and rework processes. Furthermore, due to the high content of reactive species of Sn, and the addition of Cu and In element, the interaction of the Sn–Ag–Cu–In solder with surface coatings may become more complicated. In the application of the BGA ball attach-
ment, an OSP-coated Cu bond pad is adopted not only on package substrates, but also on the PCB side. Thus, it is necessary to investigate the effect of multiple reflow process on the solder joints of this new alloy on the OSP-coated Cu bond pad. This paper aims to evaluate the Sn–Ag–Cu–In BGA solder joints on shearing strength and microstructure characterization after multiple reflow process and to compare with the popular Pb-free solder Sn–3Ag–0.5Cu, and the traditional Sn–37Pb solder.
Fig. 4. Typical fracture surfaces of the SP solder joints after 25 times reflow: (a) a whole solder pad just after shear test and (b) a magnified view of a fraction of (a) capturing IMC grains and bulk solder on the pad.
Fig. 5. Typical fracture surfaces of the SAC solder joints: (a) a low magnification view of solder pads just after the shear test and (b) magnified view of a bond pad capturing the bulk solder on the pad.
2. Experiments Three commercially available BGA solder balls of Sn–37Pb, Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–8In (hereafter SP, SAC and SACI, respectively) were used in this study. The diameter of BGA solder balls was 760 m. The melting range of the SACI solder was examined to be within 202–208 ◦ C by a differential scanning calorimetric (DSC) method. Common FR4 substrates, 1mm thick, were prepared with OSP coatings on the 30 m thick Cu bond pad. The Cu pad is 650 m in
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diameter; however, the diameter of the mask opening is about 850 m. At first, WS600 type paste flux from Alpha Metals was screen printed on the bond pad of the FR4 substrate. Afterward, the solder balls were attached onto the desired pad positions on the substrate. After that, they were put into a BTU VIP-70N model 5-zone forced convection reflow oven to carry out soldering at nitrogen atmosphere. For the sake of comparison, the temperature profile was set to be the same for all the solders and substrates. The temperature profile was measured on the surface of the substrates with a Super MOLE E31-900-45/10 wireless profiler. The peak temperature is 251 ◦ C and the times above melting point of the SP, SAC and SACI solders (183, 202 and 217 ◦ C), are about 140, 102 and 82 s, respectively. After the first reflow, all the samples were divided into different groups for subsequent reflows of 2, 4, 9, 16 and 25 times. The reflowed samples were cleaned for 5 min by alcohol in an ultrasonic machine for the preparation of shearing test and microstructure observation. The solder joint shearing test was performed using a Dage 4000 series bond tester. The shearing speed was 500 m/s and shear height of the blade was 150 m. The shearing force was an average of 32 measurements, in which the two extremes were not accounted. Another set of samples was metallographically crosssectioned, mounted in a cold setting epoxy, and polished to towards the middle of the solder joints. To reveal the thicknesses and morphologies of intermetallic compounds (IMC), the polished surfaces were slightly etched in a 3 vol.% HCl aqueous solution. At last, the fracture surfaces after shearing test and the cross-section of solder joints were viewed by a Philips XL40 FEG type scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis system.
significant change on the shear strength through the multiple reflow process. The SACI solder joints show the highest strength, whereas the SP solder joints exhibit the lowest results regardless of the reflow times. This agrees well with the empirical results that most of the Pb-free solders are stronger than the SP solders. Moreover, it is also reported that ternary solder SAC is stronger that binary Sn–Ag solder because of the addition of the Cu [22]. Addition of one more element to the SAC system may yield higher strength. The ‘In’ atoms are speculated to enter the Sn-matrix lattice sites to construct a substitutional solid solution. The In solute atoms are expected to be attracted to and interacted with both edge and screw dislocations in impending dislocation movement, which increase the strength of the solder [10]. It is also believed that the addition of ‘In’ element to the SAC solder, not only lowers the melting range, but also improves the wettability since ‘In’ is a low melting point metal and active element. Thus, it can enhance the metallurgical bonding between solder balls and pads. Therefore, significant differences in shearing strength are observed among the three solders, especially for SACI solder.
3. Results and discussion 3.1. Shearing strength The variations of average shearing force, as a function of multiple reflows of the SACI, SAC and SP solder joints are plotted in Fig. 1. It is clear that the changes of shearing forces with the increase of numbers of reflow process are not as remarkable as expected. Although shearing forces remain at a relatively stable level for each case, little changes for SACI solder joints after three times reflow and for SP solder joints after 10 times reflow should be marked down to understand the changes in the fracture surface and the microstructure. It is supposed that once the initial IMC layers are formed in the first soldering process, they can retard the extensive dissolution of the pad metals into the solder joints during the subsequent reflow treatment. It is worth mentioning that the cooling rate for each of the reflow process was identical. Thus, the microstructure of the solder joint interface as well as bulk solder does not change to an extent that would result in a
Fig. 6. Typical fracture surfaces of the SACI solder joints after one and two times reflow: (a) a portion of a solder pad just after the shearing test and (b) a magnified view the solder pad capturing the IMC and the bulk solder on the pad.
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3.2. Shearing fracture manners Fig. 2 schematically illustrates a BGA solder joint on a FR4 substrate and a shearing test setup. It should be noted that the diameter of solder mask opening is larger than that of the bond pad. This case normally occurs in the field application of the printed circuit board (PCB) industry when the pad design is not defined by the solder mask aperture or the precision of solder mask processing is not high. Since the shearing height, i.e. the distance from blade tip to solder mask surface is 150 m, the solder joint should crack at the position lower than the shearing height. Fig. 3(a) shows the typical configuration of SP solder joints just after shearing tests. The solder balls deform in large amplitude around the contact position with the blade tip. The fracture takes place just below the deformed part under the imprint of tip. Fig. 3(b) depicts the representative facture surface with an apparent ductile manner. Fig. 3(c) demonstrates the cross-sectional view, showing that a thin layer of solder remains on the top of the IMC layer. It is confirmed that up to 16 times reflow, almost all the SP solder joints separate in the bulk solder just near above the IMC layer, no matter
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what reflow times were used. The results indicate that the shearing strength of the bulk solder is weaker than the adhesion strength between solder and pad, as well as pad to the substrate. However, after 25 times reflow, the fracture mode was found to be mixed-typed as shown in Fig. 4. While major portion of the pad area is covered by solders, some Cu–Sn IMC particles are exposed out, which indicates a decreasing trend of the interfacial adhesion strength of IMC to solder compared to the bulk solder. This is partially due to the internal thermal stress built at the solder/IMC interfaces during repeated melting, inter-diffusion and solidification processes that weaken the Cu–Sn IMC to solder interface. However, the case of 25 times reflow should be set aside, since in practice, the solder joints have less chance to experience such higher number of reflow. The SAC solder joints have analogous shearing behaviors to that of the SP solder joints, as can be observed in Fig. 5. Nevertheless, the fracture surface of the SAC solder joints exhibits a relatively smooth appearance. It is reasonable that the SAC solder generates less ductile deformation during the shearing test since it is more rigid and thus has less deformation capacity when compared with the SP solder [22].
Fig. 7. Typical views of the SACI solder joints after the Cu pad sheared off from the FR4 substrates that have experienced 4, 9 and 16 times reflow: (a) the substrate side, (b) the solder joint that was sheared off from the FR4 and (c) large view of the bottom surface of the Cu pad on the solder joint sheared off.
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The SACI solder joints show diverse fracture manners with different number of reflow. For the SACI solder joints, after one and two times reflow, the shearing normally generates a mixed mode of fracture surface with a fraction of bulk solder crack and solder/IMC interface separation, as shown in Fig. 6. This finding implies that fracture propagates through the interface of IMC layer and bulk solder, which reflects higher strength of bulk SACI solder compared to the adhesion strength of IMC layer to SACI solder. However, when the reflow times surpass 4, it is rather different and most of the SACI solder joints fail at the Cu pad/rosin interface. The slight decrease in shearing force for SACI solder joint after two times reflow as seen in Fig. 1 also reflects the deterioration of the joint strength. In a typical fracture photograph, the resin and glass fiber (constituents of FR4 substrate), are visible because the whole Cu pad is peeled away, as displayed in Fig. 7(a and b), respectively. This is mainly ascribed to the degradation of the pad/resin interface caused by the multireflow process. Fig. 7(c) is a large view of the bottom surface of the pulled off Cu pad. From this finding it is clear that the strength of either bulk SACI solder or SCAI solder/IMC layer or IMC layer/Cu bond pad is stronger than Cu bond pad/FR4.
This kind of phenomena is registered neither for the SP nor for SAC solder joints. It can be deduced that the pad/resin interface degrades to a certain level during multiple reflow process, but it still stronger than the intrinsic shearing strength of the bulk solder of the SP and SAC as well as their interfaces with the IMC layer and the interfaces of IMC layer to Cu bond pad.
Fig. 8. Interfacial microstructures of the SP solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
Fig. 9. Interfacial microstructures of the SAC solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
3.3. Interfacial microstructures Figs. 8–10 show the cross-sectional photographs of the IMC developed in the three kinds of solder joints, in which 1 time and 16 times reflow are compared. In the SP solder joints, the IMC layer with one time reflow is not compact, as shown in Fig. 8(a). Although the IMC is believed to be Cu6 Sn5 [23], it is difficult to obtain an accurate composition since it is mixed with the Cu base. This reveals the shortcoming of the OSP that it does not uniformly decompose or react with the flux during the soldering. After 16 times reflow, as shown in Fig. 8(b), the IMC layer is much thicker and denser. And, the upper part of the IMC shows a loose flake-layer shape, which is the evidence of the IMC dissolution and spalling during the multiple reflow process
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Fig. 11. Variation of the mean thickness of IMC layers in the solder joints as a function of the number of reflow cycles.
Fig. 10. Interfacial microstructures of the SACI solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
[24]. The SAC solder joints also show this kind of phenomena, when comparing Fig. 9(a) with Fig. 9(b). For the SACI solder joints, it is very interesting to find huge amounts of Ag–In–Sn phases at the interfaces together with the Cu–Sn IMC layer, as shown in Fig. 10(a and b). The Ag and In atom percentage in this phase is detected to be around 71 and 13%, respectively, with only a small amount of Cu atoms (less than 1.6%) entrapped. In order to clarify the compositional evolution of the IMC between 1 and 16 times reflow, the IMC phases are denoted with numbers in the figures for the SAC and SACI solder joints. The IMC layer at the interface is given by No. 1, and the Cu–Sn IMC particles in bulk solder are registered as No. 2. The small Ag-rich IMC particles and large Ag–Inrich phase in the bulk solder are captioned as No. 3 and No. 4, respectively. It is found from EDX results that with the increasing of the number of reflow cycles, the layered IMC (No. 1) at the solder/pad interface tend to dissolve and entrap more base metal atoms (i.e., Cu). At the same time, the Cu content of the Cu–Sn IMC (No. 2) particles dispersed in bulk solder also increases a lot. The Ag content in No. 3 and No. 4 IMC only increase slightly (less than 2.0 at.%). It needs to be mentioned that there is no any fracture noticed through the
interface of solder/Ag–In–Sn although herein large amounts of Ag–In–Sn phases are clear from the microstructural characterization. On the other hand strengthening of the SACI solder because of In addition is thought be the solid solution strengthening effect of In along with the precipitation hardening of Ag–In–Sn phases in the bulk solder. In general for all the cases, the IMC layer (No. 1) grows thicker; and the IMC particles (No. 2, 3 and 4) tend to coarsen and to appear in larger quantities with increased reflow times. This suggests that there is a priority of some atoms (Cu or Ag) to nucleate and precipitate on the IMC surfaces. A total of five readings of the IMC layer thickness are counted for each case. The mean thickness of the IMC layer is plotted with respect to the reflow times, as shown in Fig. 11. The power law δ = ktn is accepted to give an approximate prediction of the experimental results of the IMC layer thickness during reflow soldering and isothermal aging [23,25–27]. More work is needed in order to validate this kinetic for the multiple reflow process. It is suggested to modify the equation in a way that both the number of reflow cycles and retarding effect and dissolution of the initially formed IMC layer can be taken into account.
4. Conclusions This work investigates the effect of multiple reflow processes on the shearing behavior and interfacial microstructures of the BGA solder joints of SP, SAC and SACI solders. The conclusions are summarized as follows. (1) For all solders, the variation of shearing forces with multiple reflow times is not as remarkable as expected. The SACI solder joints show the highest strength, whereas the SP solder joints exhibit the lowest results regardless of the reflow times.
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(2) For both the SP and SAC solder joints, irrespective of the number of reflow cycles, the fractures mainly happen in the bulk solders just near above the IMC layer. After one and two times reflow of SACI solder joints, a solder tearing together with solder/IMC separation has been noticed. Meanwhile, when the number of reflow cycles is larger than 4, the SACI solder joints always fail at the pad/resin interface, which reveals that shear strength of SACI is much higher (both for bulk solders of SACI and solder joints of SACI) compared to the case of intrinsic properties of FR4 substrates. (3) The compositional evolution of the IMC phases within the solder joints is examined. The reflow cycle facilitates diffusion of the Ag, Cu and In atoms to IMC particles thus coarsening of those IMC grains. Ag–In–Sn phase in large shapes has been found near the IMC layer within the SACI solder joints. However, there is not any detrimental effect of such large shaped IMC as has been noticed in this study.
Acknowledgements This project has been supported by CityU CERG project (9040887, CityU 1106/04E). The discussion with Mr. Chen and Mr. Xu at NUS of Singapore are gratefully acknowledged.
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Effect of multiple reflow processes on the reliability of ball grid array (BGA) solder joints W.H. Zhong a , Y.C. Chan a,∗ , M.O. Alam a , B.Y. Wu a , J.F. Guan b a
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, PR China b Department of Mineral Processing, Wuhan University of Science & Technology, Wuhan, PR China Received 26 June 2005; received in revised form 20 July 2005; accepted 22 July 2005 Available online 15 February 2006
Abstract This paper studies the mechanical behaviors and microstructures of the ball grid array (BGA) solder joints against OSP-coated Cu pads on FR4 substrates after multiple reflow process. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with the popular Pb-free solder Sn–3Ag–0.5Cu (SAC), and the traditional Sn–37Pb (SP) solder. It is found that SACI solder joints provide higher shearing strength over SAC and SP solder joints during multiple reflow. The shearing force of the three types of solder joints does not have significant change with the increase of the number of reflow cycles. The fracture of all SAC and SP solder joints mainly occurs in the bulk solder, whereas the SACI solder joints show a diverse manner, with various combinations of bulk solder facture, solder/IMC interface dissociation, and pad/resin interface failure. The differences in mechanical behaviors are interpreted in terms of diffusion and interaction, as well as chemical and thermal induced degradation. The microstructure evolutions of the solder joints during multiple reflow soldering are also examined. A continuous change of chemical compositions in the IMC layer/particles has been noticed. Large-shaped Ag–In–Sn phase near the IMC layer has been found within the SACI solder joints on the Cu pads. © 2005 Elsevier B.V. All rights reserved. Keywords: Multiple reflow; Pb-free; Ball grid array solder joints; Shearing force; Microstructure
1. Introduction Due to worldwide environmental concerns, public sentiments and market strategies, as well as governmental regulations on the use of toxic Pb in the electronic products, the conventional Sn–Pb solders are under strict scrutiny. Therefore, many Sn-rich alloy systems have been developed as alternative candidate Pb-free solders, which have attracted much researches and trial applications in recent years [1]. With combination of process attributes (modest melting point and reasonable solderability), comparable electrical performances and good mechanical properties, the Sn–Ag–Cu system is one of the potential choices recommended by some organizations, such as consortium of National Physics Laboratory (NPL), International Tin Research Institute (ITRI) ∗
Corresponding author. Tel.: +852 2788 7130; fax: +852 2788 7579. E-mail address: [email protected] (Y.C. Chan).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.047
and Department of Trade & Industry (DTI) of UK, National Electronics Manufacturing Initiative (NEMI) and National Center for Manufacturing Sciences (NCMS) of USA, as well as Japan Electronics and Information Technologies Industries Association (JEITA), etc. [2–3]. Although great effort has been made for more than 10 years to get the Pb out of electronic industry, until now, there are still some tough barriers needed to override for a widespread and uniform industrial acceptance of favorable solder alloys and matched materials plus processes [4–5]. Besides the lack of legislation, the high cost and insufficient backup data of Pb-free operation (compared to the well-established Sn–Pb soldering system) are cautious consideration nowadays. Moreover, due to the variation of the reflow temperature and materials, another challenge to massive Pb-free application is the uncertainty of many reliability issues with thermal damage, mechanical degradation, tin whisker growth and extensive solder/pad interfacial reaction.
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Fig. 2. Schematic drawing of a solder joint and shearing test setup. Fig. 1. Variation of average shearing force of solder joints as a function of the number of reflow cycles.
So, many individuals and organizations have been making efforts to promote the Pb-free implementation [6–9]. Recently, Hwang et al. has invented a novel solder alloy with the addition of In element and claimed that
Sn–Ag–Cu–In system possesses low melting temperature range, which can achieve Pb drop-off without changing the conventional reflow temperature and related materials and equipments [10]. This may cut off the overall manufacturing cost because of the less modification of the present facili-
Fig. 3. Typical shearing results of the SP solder joints: (a) low magnification image capturing two solder joints, (b) a fracture surface on the pad side and (c) a cross-sectional picture of the pad after shearing test.
W.H. Zhong et al. / Journal of Alloys and Compounds 414 (2006) 123–130
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ties and technologies. Furthermore, it can also benefit the perspective of Pb-free application in the high-demanding packages such as the flip chip (FC) and ball grid array (BGA) technology, which can reduce the reliability problems due to low-temperature process attributes. The BGA technology becomes more and more popular by the virtue of enhancing miniaturization, performance and functionality. As the high-density packaging develops towards finer pitch and smaller size, the importance of mechanical integrity of the solder joints is essentially emphasized to the reliability concerns. So far quite a lot of work on mechanical strengths of the BGA solder joints has been done based on different solders, pad and substrate design, shearing/pulling parameters, reflow ambience and aging conditions, while only a few works were focused on the effect of multiple reflow process [11–21]. However, the BGA solder joints may experience several times of reflow during component manufacturing, soldering and rework processes. Furthermore, due to the high content of reactive species of Sn, and the addition of Cu and In element, the interaction of the Sn–Ag–Cu–In solder with surface coatings may become more complicated. In the application of the BGA ball attach-
ment, an OSP-coated Cu bond pad is adopted not only on package substrates, but also on the PCB side. Thus, it is necessary to investigate the effect of multiple reflow process on the solder joints of this new alloy on the OSP-coated Cu bond pad. This paper aims to evaluate the Sn–Ag–Cu–In BGA solder joints on shearing strength and microstructure characterization after multiple reflow process and to compare with the popular Pb-free solder Sn–3Ag–0.5Cu, and the traditional Sn–37Pb solder.
Fig. 4. Typical fracture surfaces of the SP solder joints after 25 times reflow: (a) a whole solder pad just after shear test and (b) a magnified view of a fraction of (a) capturing IMC grains and bulk solder on the pad.
Fig. 5. Typical fracture surfaces of the SAC solder joints: (a) a low magnification view of solder pads just after the shear test and (b) magnified view of a bond pad capturing the bulk solder on the pad.
2. Experiments Three commercially available BGA solder balls of Sn–37Pb, Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–8In (hereafter SP, SAC and SACI, respectively) were used in this study. The diameter of BGA solder balls was 760 m. The melting range of the SACI solder was examined to be within 202–208 ◦ C by a differential scanning calorimetric (DSC) method. Common FR4 substrates, 1mm thick, were prepared with OSP coatings on the 30 m thick Cu bond pad. The Cu pad is 650 m in
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diameter; however, the diameter of the mask opening is about 850 m. At first, WS600 type paste flux from Alpha Metals was screen printed on the bond pad of the FR4 substrate. Afterward, the solder balls were attached onto the desired pad positions on the substrate. After that, they were put into a BTU VIP-70N model 5-zone forced convection reflow oven to carry out soldering at nitrogen atmosphere. For the sake of comparison, the temperature profile was set to be the same for all the solders and substrates. The temperature profile was measured on the surface of the substrates with a Super MOLE E31-900-45/10 wireless profiler. The peak temperature is 251 ◦ C and the times above melting point of the SP, SAC and SACI solders (183, 202 and 217 ◦ C), are about 140, 102 and 82 s, respectively. After the first reflow, all the samples were divided into different groups for subsequent reflows of 2, 4, 9, 16 and 25 times. The reflowed samples were cleaned for 5 min by alcohol in an ultrasonic machine for the preparation of shearing test and microstructure observation. The solder joint shearing test was performed using a Dage 4000 series bond tester. The shearing speed was 500 m/s and shear height of the blade was 150 m. The shearing force was an average of 32 measurements, in which the two extremes were not accounted. Another set of samples was metallographically crosssectioned, mounted in a cold setting epoxy, and polished to towards the middle of the solder joints. To reveal the thicknesses and morphologies of intermetallic compounds (IMC), the polished surfaces were slightly etched in a 3 vol.% HCl aqueous solution. At last, the fracture surfaces after shearing test and the cross-section of solder joints were viewed by a Philips XL40 FEG type scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) analysis system.
significant change on the shear strength through the multiple reflow process. The SACI solder joints show the highest strength, whereas the SP solder joints exhibit the lowest results regardless of the reflow times. This agrees well with the empirical results that most of the Pb-free solders are stronger than the SP solders. Moreover, it is also reported that ternary solder SAC is stronger that binary Sn–Ag solder because of the addition of the Cu [22]. Addition of one more element to the SAC system may yield higher strength. The ‘In’ atoms are speculated to enter the Sn-matrix lattice sites to construct a substitutional solid solution. The In solute atoms are expected to be attracted to and interacted with both edge and screw dislocations in impending dislocation movement, which increase the strength of the solder [10]. It is also believed that the addition of ‘In’ element to the SAC solder, not only lowers the melting range, but also improves the wettability since ‘In’ is a low melting point metal and active element. Thus, it can enhance the metallurgical bonding between solder balls and pads. Therefore, significant differences in shearing strength are observed among the three solders, especially for SACI solder.
3. Results and discussion 3.1. Shearing strength The variations of average shearing force, as a function of multiple reflows of the SACI, SAC and SP solder joints are plotted in Fig. 1. It is clear that the changes of shearing forces with the increase of numbers of reflow process are not as remarkable as expected. Although shearing forces remain at a relatively stable level for each case, little changes for SACI solder joints after three times reflow and for SP solder joints after 10 times reflow should be marked down to understand the changes in the fracture surface and the microstructure. It is supposed that once the initial IMC layers are formed in the first soldering process, they can retard the extensive dissolution of the pad metals into the solder joints during the subsequent reflow treatment. It is worth mentioning that the cooling rate for each of the reflow process was identical. Thus, the microstructure of the solder joint interface as well as bulk solder does not change to an extent that would result in a
Fig. 6. Typical fracture surfaces of the SACI solder joints after one and two times reflow: (a) a portion of a solder pad just after the shearing test and (b) a magnified view the solder pad capturing the IMC and the bulk solder on the pad.
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3.2. Shearing fracture manners Fig. 2 schematically illustrates a BGA solder joint on a FR4 substrate and a shearing test setup. It should be noted that the diameter of solder mask opening is larger than that of the bond pad. This case normally occurs in the field application of the printed circuit board (PCB) industry when the pad design is not defined by the solder mask aperture or the precision of solder mask processing is not high. Since the shearing height, i.e. the distance from blade tip to solder mask surface is 150 m, the solder joint should crack at the position lower than the shearing height. Fig. 3(a) shows the typical configuration of SP solder joints just after shearing tests. The solder balls deform in large amplitude around the contact position with the blade tip. The fracture takes place just below the deformed part under the imprint of tip. Fig. 3(b) depicts the representative facture surface with an apparent ductile manner. Fig. 3(c) demonstrates the cross-sectional view, showing that a thin layer of solder remains on the top of the IMC layer. It is confirmed that up to 16 times reflow, almost all the SP solder joints separate in the bulk solder just near above the IMC layer, no matter
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what reflow times were used. The results indicate that the shearing strength of the bulk solder is weaker than the adhesion strength between solder and pad, as well as pad to the substrate. However, after 25 times reflow, the fracture mode was found to be mixed-typed as shown in Fig. 4. While major portion of the pad area is covered by solders, some Cu–Sn IMC particles are exposed out, which indicates a decreasing trend of the interfacial adhesion strength of IMC to solder compared to the bulk solder. This is partially due to the internal thermal stress built at the solder/IMC interfaces during repeated melting, inter-diffusion and solidification processes that weaken the Cu–Sn IMC to solder interface. However, the case of 25 times reflow should be set aside, since in practice, the solder joints have less chance to experience such higher number of reflow. The SAC solder joints have analogous shearing behaviors to that of the SP solder joints, as can be observed in Fig. 5. Nevertheless, the fracture surface of the SAC solder joints exhibits a relatively smooth appearance. It is reasonable that the SAC solder generates less ductile deformation during the shearing test since it is more rigid and thus has less deformation capacity when compared with the SP solder [22].
Fig. 7. Typical views of the SACI solder joints after the Cu pad sheared off from the FR4 substrates that have experienced 4, 9 and 16 times reflow: (a) the substrate side, (b) the solder joint that was sheared off from the FR4 and (c) large view of the bottom surface of the Cu pad on the solder joint sheared off.
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The SACI solder joints show diverse fracture manners with different number of reflow. For the SACI solder joints, after one and two times reflow, the shearing normally generates a mixed mode of fracture surface with a fraction of bulk solder crack and solder/IMC interface separation, as shown in Fig. 6. This finding implies that fracture propagates through the interface of IMC layer and bulk solder, which reflects higher strength of bulk SACI solder compared to the adhesion strength of IMC layer to SACI solder. However, when the reflow times surpass 4, it is rather different and most of the SACI solder joints fail at the Cu pad/rosin interface. The slight decrease in shearing force for SACI solder joint after two times reflow as seen in Fig. 1 also reflects the deterioration of the joint strength. In a typical fracture photograph, the resin and glass fiber (constituents of FR4 substrate), are visible because the whole Cu pad is peeled away, as displayed in Fig. 7(a and b), respectively. This is mainly ascribed to the degradation of the pad/resin interface caused by the multireflow process. Fig. 7(c) is a large view of the bottom surface of the pulled off Cu pad. From this finding it is clear that the strength of either bulk SACI solder or SCAI solder/IMC layer or IMC layer/Cu bond pad is stronger than Cu bond pad/FR4.
This kind of phenomena is registered neither for the SP nor for SAC solder joints. It can be deduced that the pad/resin interface degrades to a certain level during multiple reflow process, but it still stronger than the intrinsic shearing strength of the bulk solder of the SP and SAC as well as their interfaces with the IMC layer and the interfaces of IMC layer to Cu bond pad.
Fig. 8. Interfacial microstructures of the SP solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
Fig. 9. Interfacial microstructures of the SAC solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
3.3. Interfacial microstructures Figs. 8–10 show the cross-sectional photographs of the IMC developed in the three kinds of solder joints, in which 1 time and 16 times reflow are compared. In the SP solder joints, the IMC layer with one time reflow is not compact, as shown in Fig. 8(a). Although the IMC is believed to be Cu6 Sn5 [23], it is difficult to obtain an accurate composition since it is mixed with the Cu base. This reveals the shortcoming of the OSP that it does not uniformly decompose or react with the flux during the soldering. After 16 times reflow, as shown in Fig. 8(b), the IMC layer is much thicker and denser. And, the upper part of the IMC shows a loose flake-layer shape, which is the evidence of the IMC dissolution and spalling during the multiple reflow process
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Fig. 11. Variation of the mean thickness of IMC layers in the solder joints as a function of the number of reflow cycles.
Fig. 10. Interfacial microstructures of the SACI solder joints: (a) after 1 time reflow and (b) after 16 times reflow.
[24]. The SAC solder joints also show this kind of phenomena, when comparing Fig. 9(a) with Fig. 9(b). For the SACI solder joints, it is very interesting to find huge amounts of Ag–In–Sn phases at the interfaces together with the Cu–Sn IMC layer, as shown in Fig. 10(a and b). The Ag and In atom percentage in this phase is detected to be around 71 and 13%, respectively, with only a small amount of Cu atoms (less than 1.6%) entrapped. In order to clarify the compositional evolution of the IMC between 1 and 16 times reflow, the IMC phases are denoted with numbers in the figures for the SAC and SACI solder joints. The IMC layer at the interface is given by No. 1, and the Cu–Sn IMC particles in bulk solder are registered as No. 2. The small Ag-rich IMC particles and large Ag–Inrich phase in the bulk solder are captioned as No. 3 and No. 4, respectively. It is found from EDX results that with the increasing of the number of reflow cycles, the layered IMC (No. 1) at the solder/pad interface tend to dissolve and entrap more base metal atoms (i.e., Cu). At the same time, the Cu content of the Cu–Sn IMC (No. 2) particles dispersed in bulk solder also increases a lot. The Ag content in No. 3 and No. 4 IMC only increase slightly (less than 2.0 at.%). It needs to be mentioned that there is no any fracture noticed through the
interface of solder/Ag–In–Sn although herein large amounts of Ag–In–Sn phases are clear from the microstructural characterization. On the other hand strengthening of the SACI solder because of In addition is thought be the solid solution strengthening effect of In along with the precipitation hardening of Ag–In–Sn phases in the bulk solder. In general for all the cases, the IMC layer (No. 1) grows thicker; and the IMC particles (No. 2, 3 and 4) tend to coarsen and to appear in larger quantities with increased reflow times. This suggests that there is a priority of some atoms (Cu or Ag) to nucleate and precipitate on the IMC surfaces. A total of five readings of the IMC layer thickness are counted for each case. The mean thickness of the IMC layer is plotted with respect to the reflow times, as shown in Fig. 11. The power law δ = ktn is accepted to give an approximate prediction of the experimental results of the IMC layer thickness during reflow soldering and isothermal aging [23,25–27]. More work is needed in order to validate this kinetic for the multiple reflow process. It is suggested to modify the equation in a way that both the number of reflow cycles and retarding effect and dissolution of the initially formed IMC layer can be taken into account.
4. Conclusions This work investigates the effect of multiple reflow processes on the shearing behavior and interfacial microstructures of the BGA solder joints of SP, SAC and SACI solders. The conclusions are summarized as follows. (1) For all solders, the variation of shearing forces with multiple reflow times is not as remarkable as expected. The SACI solder joints show the highest strength, whereas the SP solder joints exhibit the lowest results regardless of the reflow times.
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(2) For both the SP and SAC solder joints, irrespective of the number of reflow cycles, the fractures mainly happen in the bulk solders just near above the IMC layer. After one and two times reflow of SACI solder joints, a solder tearing together with solder/IMC separation has been noticed. Meanwhile, when the number of reflow cycles is larger than 4, the SACI solder joints always fail at the pad/resin interface, which reveals that shear strength of SACI is much higher (both for bulk solders of SACI and solder joints of SACI) compared to the case of intrinsic properties of FR4 substrates. (3) The compositional evolution of the IMC phases within the solder joints is examined. The reflow cycle facilitates diffusion of the Ag, Cu and In atoms to IMC particles thus coarsening of those IMC grains. Ag–In–Sn phase in large shapes has been found near the IMC layer within the SACI solder joints. However, there is not any detrimental effect of such large shaped IMC as has been noticed in this study.
Acknowledgements This project has been supported by CityU CERG project (9040887, CityU 1106/04E). The discussion with Mr. Chen and Mr. Xu at NUS of Singapore are gratefully acknowledged.
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